1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) modulation makes an efficient use of its radio spectrum by placing modulated subcarriers as close as possible without causing Inter-Carrier Interference (ICI). OFDM modulation has been adopted in various standards, most notably digital audio broadcast (DAB), digital video broadcast (DVB), asymmetric digital subscriber line (ADSL), IEEE LAN (802.11a and 802.11g) and IEEE MAN 802.16a. OFDM modulation is also being considered for various next generation wireless standards.
The selection of subcarriers used by each cell in a wireless system has significant ramifications regarding the rate at which data can be transmitted to a mobile station. For example, generally, the greater the number of subcarriers, the higher the data rate that may be supported. However, as the number of subcarriers increases, the amount of power that may be allocated to a subcarrier is generally reduced. For example, if eight subcarriers are employed to transmit data, then only about ⅛ of the total power is typically allocated to each subcarrier. For mobile stations with high signal to noise ratios (SNRs), such as those near a transmitting base station, these low power signals may be adequate, and relatively high data rates may be acheived. However, for mobile stations that are more distant or are subject to interference from adjacent cells, some or all of the subcarriers may not have sufficient power to reliably deliver data signals to the mobile stations.
The low SNRs experienced by the “weak” mobile stations may be overcome by reducing the number of subcarriers and allocating more power to each of the remaining subcarriers. For example, if only two subcarriers are used, then ½ of the available power may be allocated to each of the subcarriers. The higher power will normally result in improved delivery of data to the “weak” mobile stations, but will result in lower data rates for the “strong” mobile stations.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.